You said, "I would much prefer to regard the damage wrought by Columbia as contributory, not causal." The fact of the matter is, the Humans' actions were contributory, not causal. The actions of the Humans inadvertently created the circumstances in which Sedín took them as her first victims. But no one forced Sedín to choose to "consume" her fellow Caeliar and to then possess the Humans. That was the result of a combination of choices that Sedín made while still psychologically stable and then actions she took while "brain damaged," so to speak.
